Method for diffusing b or p into s: substrates

ABSTRACT

A method is disclosed whereby a semiconductor silicon substrate wafer is diffused with a P or N type dopant or &#39;&#39;&#39;&#39;impurity&#39;&#39;&#39;&#39; in an open tube at a temperature of about 1050*C in the presence of at least a 50:1 ratio of water to dopant whereby the surface concentration of impurity is controlled below the solid solubility of the dopant in silicon.

United States Patent [191 Demsky et al. 1 Nov. 6, 1973 [54] METHOD FORDIFFUSING B OR P INTO 5: 3,477,887 ll/l969 Ehlenberger 148/189SUBSTRATES 3,484,314 12/1969 Bohne et al 148 188 3,442,725 5/1969Huffman et al [48/189 [75] Inventors: Herbert M. Demsky, WappingersFalls; Wilbur H. Dexter, Hyde Park; Reginald F. Lever, Putnam Valley;Primary Ozaki g g Newburgh f Attorney-Hanifin and Jancin and Daniel E.lgo

[73] Assignee: International Business Machines Corporation, Armonk, NY.221 Filed: Apr; 14, 1971 [57] ABSTRACT Appl' 135009 A method isdisclosed whereby a semiconductor silicon substrate wafer is diffusedwith a P or N type dopant or [52] US. Cl 148/189, 148/186, 148/187 p yin an p tube at a temperature of about [51] Int. Cl. H01! 7/44 05 i hpresence of at l ast a 50:1 ratio of water [58] Field of Search 148/l89,186, 187 to dopant wh y h rf concentration of impurity is controlledbelow the solid solubility of the dop- [56] References Cited ant inSilicon.

UNITED STATES PATENTS 2,802,760 8/1957 Derick et a]. .L 148/189 2Claims, 4 Drawing Figures PAIENIEDuuv s 1975 sum 1 or 2 F. m N 0 Z w 0 GN F I K M MM 0 B L M A N R U S 1 R F H IL A N w w G I s I F F E m N 0 ZT 0. H 'll R E A L 01.. G T ZB R F. B R .H W E U m 0 s T m A N 6 B T P R0 rr. a B m INERT GAS DILUTION INLET FIG. 2

INVENTORS HERBERT M. DEMSKY PARTIAL PRESSURE 'HZO-(ATMOSPHERES) WILBURH. DEXTER REGINALD F. LEVER PAUL A. MELZ mommy PATENTEDHDV 6 mm SHEET 26F 2 FIG. 3

BBQ, PARTIAL PRESSURE -(ATMOSPHERES) If: 82 x 10 TIME (M|NUTES) a$250228 GE;

FIG; 4

METHOD FOR DIFFUSING B OR P INTO S: SUBSTRATES BACKGROUND OF THEINVENTION In the manufacture of semiconductor devices, it is necessaryto introduce certain impurities or dopants into semiconductor materialin order to form P-N junctions. The prior art has taught certain methodsfor accomplishing these diffusions. It has been the practice to exposethe etched surface of semiconductor material at an elevated temperatureto a vapor which includes the impurity to be diffused. In the formationof a P-type layer in an N-type silicon, BBr and B I-I materials havebeen introduced into a furnace in the presence of oxygen as the materialcontaining the boron impurity to be diffused. Similarly, in theformation of an N-type layer in P-type silicon lCl POCl and IP have beenintroduced into a furnace in the presence of oxygen as the materialcontaining the phosphorus to be diffused.

It has been the practice to first predeposit the dopant upon the surfaceof a semiconductor at a first temperature level followed by a second andelevated temperature for actual diffusion. Likewise, where if variousjunction depths are desired, more than one diffusion operation isrequired. Another known method comprises the steps of forming anazeotropic mixture of the dopant and water, vaporizing the mixture andcarrying it onto the semiconductor surface by means of a carrier gas anddiffusing the dopant from said mixture into the semiconductor material.

Diffusion of impurities produces an impurity distribution in thesemiconductive substrate material. This impurity distribution is relatedto other steps in the manufacture and fabrication and to ultimatecomponent product. A uniform concentration during impurity diffusioninto a body of semiconductor material is desirable and important in thecreation of .a plurality of devices in a single body material with otherunitary circuit structures.

Other methods provide a method whereby a first semiconductor body isdoped by heating in an evacuated system with another large body ofsemiconductor material containing the desired diffusion impurity. Thismethod has proved difficult to carry out and control. Vacuum diffusionsystems are carried out in quartz or similar material which are notcapable of being reused for subsequent diffusions.

It has been proposed to dope semiconductor bodies with boron through theuse of boron bromide and oxygen wherein boron and oxygen are introducedseparately into a furnace containing semiconductor bodies. In such amethod, a chemical reaction between the boron bromide and oxygen takesplace within the furnace producing boron oxide (B 0 and bromine. Thisreaction produces corrosive by-products and requires critical flow rateswhich contributes to non-uniform diffusions. A boron-rich borosilicateglass forms on the silicon wafer surface with a consequent heavy surfacedoping of the silicon. Also, undesirable boron oxide tends to form onthe walls of the furnace causing additional process difficulties. Thesolid solubility limit (6 X 10 atoms/cm) of boron in silicon isessentially the only concentration of boron in silicon attainable bythis method. Variable concentration is practically unattainable.

Similarly, a boron-silicon phase which is not soluble in oxide etches isformed. This condition and high surface concentration (C requires afurther thermal oxidation or drive-in step following the diffusion ifstandard photoresist technology is to be applicable with lower surfaceconcentrations of dopant. Likewise, special elevated temperature etchesare required to re move said boron-silicon phase if an ultimate uniformdiffusion is desired. Corrosive attack on the quartztube is to such anextent in most cases as to make the reaction or diffusion tubenon-reusable. Open tube diffusion of boron and phosphorus in silicon arenormally accomplished and carried out at a relatively low temperature(900-975) and short cycle in order to avoid silicon pitting. Therefore,a subsequent drive-in step is required in a separate furnace ifdiffusion results comparable to the vacuum capsule are to be obtained.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod for open tube diffusions in silicon whereby variableconcentrations of dopant are readily attainable.

A further object is a silicon diffusion method capable of producinguniform variable diffusion concentrations.

A still further object is to provide a silicon diffusion process wherebydiffusion is carried outin an open tube at essentially atmosphericpressure.

Another object of the invention is to provide a method wherebysubsequent post diffusion thermal oxidation or drive-in steps areunnecessary.

A still further objective of the invention is a method for the selectivediffusion of boron or phosphorus in silicon in a manner whereby thediffused impurity concentration is variable from the solid solubilitylimit to lower decreased concentrations.

The foregoing and other objects as well as the operation of thisinvention will become more apparent upon consideration of the followingdetailed description and illustrated by the examples and theaccompanying drawings where FIG. I is a diagrammatic representation ofa. typical conventional open tube diffusion apparatus.

FIG. 2 is a graphic representation showing the surface concentration ofimpurity or dopant (C in atoms per cubic centimeter versus partialpressure of process water in atmospheres in accordance with disclosedprocess.

FIG. 3 is a graphic illustration of surface concentration of impurity ordopant (C in atoms per cubic centimeter in a silicon wafer versuspartial pressure of dopant (boron tribromide BlBr FIG. 4 is a graphicrepresentation of sheet conductance (G, X 1000 (Ml-Io of semiconductorsilicon wafer substrates boron tribromide (BBQ) diffused or doped versustime of feed of dopant and water into the hot zone of the difiusiontube.

In general, this invention is especially suited to the formation of oneor more P-N or N-P junctions in a semiconductive body for use in deviceapplications and .comprises diffusing an N or P impurity in silicon inaccordance with the following steps:

a. heating a silicon substrate at atmospheric pressure to a temperaturebetween l000l C.

b. subjecting the heated silicon substrate to an ambient vapor flow of Nor P type impurity and water for a period of from 50 to l30 minutes.

DETAILED DESCRIPTION In a conventional open tube diffusion apparatuscomprising a quartz tube in a suitable furnace capable of heating above1200C, clean silicon semiconductor wafers are inserted in the quartztube and heated to a temperature of between 900-1200C in an ambientargon or similar inert gas atmosphere. (A diffusion temperature of about1050C is believed to be optimum). Upon reaching temperature equilibriumwith furnace atmosphere, separate streams of boron tribromide orphosphorus oxytrichloride and water are injected into hot zoneatmosphere surrounding the silicon wafer. The stream of water and BBrdopant mix and react in the immediate hot zone atmosphere surroundingthe silicon wafers. The resulting reaction is:

BBr 21-1 1-1130 3HBr The use of oxygen and an inert gas carrier producesthe reaction:

4B1? 30 28 0 6Br The presence of 13 0 is undesirable because B 0 tendsto precipitate and coats the wafers and diffusion apparatus. The firstreaction has the advantage of allowing to exist considerable partialpressures of H80 without depositing B 0 by the reaction 2HBO 13 0 H 0.Partial pressure of 13 0 can be made considerably lower than the valuein equilibrium with liquid B 0 consequently, a more dilute borosilicateglass may be grown on the wafer surface. This obviates the necessity ofa subsequent drive-in step. Similarly, by varying the borontribromide-water ratio, one is capable of producing a wide range ofoxidation rates and varying surface concentrations (C which is believedto be dependent upon oxidation rates. This allows for a processflexiblity heretofore unknown.

In order to obtain the maximum advantage of the process, the dopant, forexample BBr and water must be admixed in the furnace hot zone becauseBBr readily hydrolyzes to form undesirable B 0 if mixture with watertakes place, for example, at normal room atmospheric temperature.

It is now possible in accordance with this process to have a diffusedsurface concentration significantly below the solid solubility limit ofboron in silicon by means of a BlBr deposition. Furthermore, the oxideformed during the deposition is totally soluble in HF and the thininsoluble interface layer of oxide formed with conventional BBrdepositions is not present.

The word dopant or impurity as used herein is intended to includematerials which act as donors and acceptors in silicon substratesemiconductor wafers, for example, boron and phosphorus. The followingspecific examples are set forth, not in limitation of this invention,but in explanation and to further illustrate the applications andadvantages thereof:

EXAMPLE 1 Clean silicon wafers were placed in a conventional ladder boatcontained in an open quartz diffusion tube in a muffle type electricfurnace and heated in an argon atmosphere to 1050C. Boron tribromide andwater were separately introduced in the hot zone of the said diffusiontube at a flow rate of 55 cubic centimeters per minute at 1C carried bya suitable flow of argon for the boron tribromide and a rate of 3350cubic centimeters per minute at 23C for water. Argon dilution flow forBBr and H 0 was l1,600 cubic centimeters per minute. This flow andtemperature were maintained for a period of 120 minutes whereupon theflow of boron tribromide and water was discontinued and the tube purgedwith a continued five-minute flow of argon.

Silicon wafers were removed and found to have a 2000A. borosilicateglass film on the surface. Said film was insoluble in HF. Sheetresistivity was 8.5 ohms per square and the junction depth X was 0.056mils. Impurity or dopant surface concentration, C was 5.5 X 10 atoms percubic centimeter.

EXAMPLE ll Similar conditions as set forth in Example 1 except thatboron tribromide flow was reduced to 5 cubic centimeters per minute at1C. The silicon wafers had the following electrical properties:

Sheet resistivity (P,,) 1800 ohms per square Diffusion depth (X 0.029mils Surface concentration -(C,,) 1 X 10" atoms per cubic centimeterEXAMPLE Ill Similar conditions as delineated in Example I except borontribromide flow was controlled at 30 cubic centimeters per minute at 1Cand the water flow maintained at 3350 cubic centimeters per minute at23C. The wafers processed in this manner exhibited the followingelectrical properties:

Sheet resistivity -(P,,) ohms per square Diffusion depth --(X,) 0.048mils Surface concentration -(C,,) 4.5 X 10 atoms per cubic centimeterThe film of borosilicate glass formed on the waters was about 700A.thick and soluble in HF.

EXAMPLE IV Again, clean silicon wafers were placed in a conventionalladder boat contained in an open quartz diffusion tube in a muffle typeelectric furnace and heated in an argon atmosphere to 1050C forapproximately 5 minutes. Separately, phosphorus oxytrichloride (POCl wasfed into the wafer hot zone area at a rate of 16 cubic centimeters perminute accompanied by argon dilution of about 4600 cubic centimeters perminute. Water was separately bled into the hot zone area at about 40cubic centimeters per minute. This flow was maintained for 60 minutes,followed by a 5-minute argon flush or vent flow at about 4400 cubiccentimeters per minute. The wafers possessed the following electricalproperties:

Sheet resistivity (P,) 300 ohms per square Diffusion depth (X 0.025 milsSurface concentration (C 8 X 10 atoms per cubic centimeter It isapparent from the foregoing examples that the water to dopant orimpurity ratio is in the magnitude of from 50:1 to 500:1.

The disclosed process possesses the significant advantage being able toproduce, in essentially one step, a doped silicon substrate wafer havinga surface concentration of dopant below the solid solubility limit ofthe impurity in the silicon. This obviates the necessity of carrying outsubsequent drive-in" or oxidation steps.

It is apparent from the graphic representations in FIGS. 2, 3 and 4 thatsurface concentration (C of dopant in the silicon wafer substrate isreadily control- I 6 lable in the disclosed method by coordinating timeand selected from the group consisting of boron tribroflow rates.Electrical properties of the doped wafers are mide and phosphorusoxytrichloride and water in controlled as illustrated by FIG. 4 whereinsheet resisclose proximity to said heated substrate for a specitivity ispredictable through time and dopant feed regufied time, and lation. 5 c.cooling to room temperature.

While this invention has been particularly described 2. A method fordiffusing a boron or phosphorus conwith reference to the preferredembodiments thereof, ductivity type dopant in a silicon semiconductorsubit will be understood by those skilled in the art that the stratecomprising foregoing and other changes in form and detail may be a.heating said silicon semiconductor substrate to a made therein withoutdeparting from the spirit and 10 temperature between 950C and I050C atatmoscope of the invention. spheric pressure, and

We claim: b. separately introducing a 1:50 ratio by weight of a l. Amethod for diffusing a boron or phosphorus conconductivity type dopantselected from the group ductivity type dopant in a silicon semiconductorsubconsisting of boron tribromide and phosphorus oxstrate comprising:ytrichloride and water in close proximity to said a. heating saidsilicon semiconductor substrate to a heated substrate for a periodbetween SOg d 13 0 temperature between 9009C and l200C at atmominutes,and spheric pressure, and c. cooling to room temperature. b. separatelyintroducing a conductivity type dopant

2. A method for diffusing a boron or phosphorus conductivity type dopantin a silicon semiconductor substrate comprising a. heating said siliconsemiconductor substrate to a temperature between 950*C and 1050*C atatmospheric pressure, and b. separately introducing a 1:50 ratio byweight of a conductivity type dopant selected from the group consistingof boron tribromide and phosphorus oxytrichloride and water in closeproximity to said heated substrate for a period between 50 and 130minutes, and c. cooling to room temperature.